Corneal ulcers in animals
The collagen fibers cross the full diameter of the cornea in a strictly parallel fashion and allow 99 percent of the light to pass through without scattering.
[1] The epithelium holds the tear film in place and also prevents water from invading the cornea and disrupting the collagen fibers.
However, larger or deeper ulcers often require the presence of blood vessels to supply inflammatory cells.
Central ulcers are typically caused by trauma, dry eye, or exposure from facial nerve paralysis or exophthalmos.
Ulcers in the inferior nasal cornea may be caused by foreign material trapped under the third eyelid.
[3] Corneal ulcers are painful due to nerve exposure, and can cause tearing, squinting, and pawing at the eye.
An axon reflex may be responsible for uveitis formation — stimulation of pain receptors in the cornea results in release inflammatory mediators such as prostaglandins, histamine, and acetylcholine.
[4] Diagnosis is through direct observation of the ulcer with the use of fluorescein stain, which is taken up by exposed corneal stroma and appears green (see photos above and below).
Topical corticosteroids and anesthetics should not be used on any type of corneal ulcer because they prevent healing and will often make them worse.
The study is being conducted by board certified veterinary ophthalmologists and has shown promise in healing refractory ulcers that have failed conventional treatment .
Refractory corneal ulcers are most commonly seen in middle aged or older dogs and often occur in the other eye later.
[5] Other medications have been shown to be useful in topical treatment of refractory ulcers, including glycosaminoglycans such as sodium hyaluronate[8] and chondroitin sulfate,[9] aminocaproic acid,[10] and acetylcysteine.
Corneal ulcers in cats can be caused by trauma, detergent burns, infections, and other eye diseases.
Treatment is with topical antiviral drugs and antibiotics, and oral L-lysine, which competes with arginine and inhibits viral replication.
A distinct cause for initial ulceration is not commonly found, although in many cases it can be assumed to be traumatic in origin.
Exposure keratitis (inflamed cornea) can occur in the horse, most commonly secondary to facial nerve paralysis.
Foreign bodies embedded in the palpebral conjunctiva or the nictitating membrane can cause persistent irritation and ulceration.
Cotton swabbing is often inadequate, and corneal scraping, for example, with the blunt side of a scalpel blade is usually required.
Much of the pain associated with corneal ulceration is due to the secondary uveitis and miosis, and effective relief can often be gained with topical atropine(1%), leading to mydriasis.
Pain is also associated with inflammatory response occurring in the adjacent sclera and conjunctiva, and systemic analgesia in the form of NSAIDs is usually indicated, for example, flunixin meglumin 1.1 mg/kg, BID.
Topical NSAIDs are available (diclofenac, flurbiprofen) and effective, but have been shown to increase corneal healing time.
In horses that are difficult to treat, or in cases that require very frequent treatment, then placement of a sub-palpebral lavage system can be very useful.
Bacterial pathogens (especially Pseudomonas and β-haemolytic Streptococcus) induce the corneal epithelial cells and resident leucocytes to upregulate pro-inflammatory, and MMP-activating cytokines (IL-1,-6 and -8).
The combination of exogenous, and upregulated endogenous, proteinases leads to a rapid breakdown of collagen, with the characteristic melting appearance.
Effective antibiosis is also paramount, and gentamicin is a good empirical choice (although there are some reports of gentamicin-resistant Pseudomonas species).
